1. Field of the Invention
This invention relates to control circuits for supplying current to a load in general and more particularly to a digital control circuit for the armature winding of a direct current electric motor.
2. Description of the Prior Art
The Ward Leonard system has been utilized for many years to supply armature current to direct current electric motors. In recent years, the thyristor type of static converter has been proposed as a substitute for the Ward Leonard system as the reliability of the thyristor converter has increased and the cost has decreased. Where a three-phase alternating current source is available, a first form of unidirectional static converter includes six silicon controlled rectifiers wherein each pair of SCR's is connected in a back-to-back parallel relationship between one of the power source lines and the armature winding. A reversible field control may be utilized to effect reverse rotation of the motor. However, this form of motor control has the disadvantage of slow response time when the direction of rotation is reversed since larger horsepower motors generally have relatively large field time constants. This disadvantage may be overcome by utilizing field forcing.
In a second type of controller, reversing switches are connected between the SCR's and the armature so that the armature current may be reversed to change the direction of the motor. This type of controller has the advantage of a faster current reversal time than the first type of controller, but this controller also has the disadvantage of requiring mechanical reversing switches which are more susceptible to failure than static elements. A third type of controller is known as an anti-parallel converter and has double the number of SCR's to provide for armature current flow in either direction for changing the direction of rotation of the motor. This type of controller has the disadvantage of being more expensive than either of the first mentioned types of controllers.
Generally, in all these types of controllers, the firing signals for the SCR gates are developed using one of two methods. The first method is the pulse control method wherein a firing pulse is applied to the SCR gate when it is desired to turn the SCR on. These pulse control circuits may include a unijunction transistor relaxation oscillator or a magnetic trigger which applies a voltage to the SCR gate sufficient to turn on the SCR with reference to a point on the wave form of the power source. The magnetic trigger circuit may include a magnetic amplifier or a saturable reactor.
The second method of SCR firing is the phase shift method. The power source wave form is applied to a resistance-capacitance or a resistance-inductance network connected to the gate of the SCR. The network phase shifts the gate voltage wave form with respect to the power source wave form applied to the SCR anode, so that the portion of the power source wave form which is passed by the SCR can be determined by selecting the values of the network elements.
However, both the pulse control method and the phase shift method utilize the zero crossing point of the power source wave form as a reference point from which the firing angle is determined. Therefore, it is difficult to match the firing angles of a plurality of SCR's in a controller since each circuit element has a tolerance on its initial value and the values change as the elements age. In a motor control where the actual armature current is being compared with a current command signal, a change in either the command signal or the armature current may require several cycles before a stable state is reached, since the firing command which is generated may be correct for some of the SCR's but not correct for others as element values are mismatched or change with age.